Summary Energy is required to fuel every aspect of the stress response. While stress and energy are intertwined, little is known about the role of the cellular powerhouse ? the mitochondria ? in regulating stress ?perception?, and the resulting psycho-physiological responses. From a health perspective, this is particularly important since appropriate (de)activation of the HPA (hypothalamic-pituitary-adrenal) axis, autonomic nervous system, and inflammatory responses have been linked cardiovascular, neurodegenerative, and metabolic diseases when dysregulated. Recent work by our group has identified mitochondria as a major regulator of multi-systemic stress responses. In a study of transgenic mice with either nuclear or mitochondrial DNA (mtDNA) defects, we found that perturbing mitochondrial functions caused abnormal HPA axis glucocorticoid secretion, altered stress-reactive catecholamine levels epinephrine (E) and norepinephrine (NE), and increased levels of the pro- inflammatory cytokine IL-6 following a single exposure of restraint stress, a model of acute psychological stress in rodents. We now aim to translate these findings in humans by assessing the psychophysiological stress responses of individuals with equivalent pathogenic mtDNA defects, in comparison to spousal controls. This work is possible due to our unique access to a substantial clinical population of patients with mitochondrial disorders in our clinic. Participants will be exposed to the socially-evaluated cold pressor task (SECPT) that robustly activates the major stress response systems. As in our preclinical work, we will employ repeated measurements of multiple physiological systems to capture the kinetics of key biomarkers during stress. Specifically, we aim to (1) Determine the effects of mitochondrial dysfunction on stress-induced cortisol, E/NE, heart rate variability, and IL-6 levels in groups of individuals with known mtDNA defects; (2) Use a novel mitochondrial phenotyping platform to examine the relationship between mitochondrial health indices from blood, and summary measures of stress reactivity/recovery across patients and controls, allowing us to ascertain whether inter-individual differences in mitochondrial functions can predict stress responses; and (3) Explore the associations between psychosocial factors and the above mentioned indices of stress reactivity/recovery and mitochondrial functioning. If these preliminary data confirmed that our preclinical data translate to humans, this would provide the foundation for a full-size study to establish mitochondria as a novel sub-cellular determinant of the stress response. Such innovation could foster new ways to intervene and prevent adverse cardiovascular, cognitive, and metabolic consequences of chronic stress.